1. Field of the Invention
The present invention relates to a deflection circuit which prevents an image displayed on a picture tube from being swung by a change of a D.C. component in a deflection current when a channel is switched.
2. Description of the Prior Art
Japanese Patent Application Kokai (Laid-Open) No. 114113/75 discloses a deflection circuit comprising a sawtooth wave generator circuit including a capacitor, a current source for charging or discharging the capacitor at a constat rate during a scan period to produce a voltage of a predetermined gradient across the capacitor and a switch adapted to be closed during a retrace period to restore the capacitor voltage to an initial charged voltage at the start of scan, a deflection coil, and an amplifier circuit for amplifying a sawtooth wave voltage from the sawtooth wave generator circuit to supply a sawtooth wave current to the deflection circuit, wherein in order to prevent an image from being swung by the change of a D.C. component in the sawtooth wave current flowing in the deflection coil, which change is caused by the change of a power supply voltage to the sawtooth wave generator circuit, the change of the power supply voltage is detected to produce a voltage which is opposite in phase to the change of the D.C. component of the sawtooth wave voltage and the opposite phase voltage is combined with the sawtooth wave voltage to cancel the change of the D.C. component due to the change of the power supply voltage.
In the sawtooth wave generator circuit shown in the above reference, a maximum (or minimum) value of the sawtooth wave voltage developed across the capacitor is fixed to a predetermined voltage while a minimum (or maximum) value depends on a period during which the constant current is supplied to the capacitor from the current source, that is, a deflection frequency. Accordingly, the D.C. component of the sawtooth wave voltage changes not only with the change of the power supply voltage but also with the change of the deflection frequency.
The deflection frequency is determined by an oscillation frequency of the deflection circuit and it changes depending on whether the sawtooth wave generator circuit is synchronized with a synchronizing signal to oscillate at the frequency of the synchronizing signal, or the synchronizing signal is absent and the sawtooth wave generator circuit oscillates at a free-running frequency. Accordingly, when the circuit goes through an absence-of-synchronizing signal period such as channel switching period, the oscillation frequency of the sawtooth wave generator circuit and hence the deflection frequency changes from the synchronizing signal frequency to the free-running frequency and back to the synchronizing signal frequency as a result, the D.C. component of the sawtooth wave voltage changes.
It is apparent that the change of the D.C. component leads to the change of the deflection current where the sawtooth wave generator circuit is D.C.-coupled to the deflection circuit, and even when the sawtooth wave generator circuit is A.C.-coupled to the deflection coil, a transitional change in the deflection current occurrs, which leads to the swing of the image.
In many cases, the vertical deflection circuit vertically oscillates at a frequency of 0.5-3 Hz. This frequency is determined by a charge/discharge time constant of a capacitor used in an amplifier circuit which couples the sawtooth wave generator circuit and the deflection circuit and a D.C. blocking capacitor connected between the amplifier circuit and the deflection coil. Since this time constant is larger than the vertical period, the change of the oscillation frequency of the sawtooth wave generator circuit is delayed in time and appears as the change of the D.C. component of the vertical deflection current. When the period of free-running oscillation lasts for more than 20-120 vertical periods, the change on the image is sufficiently large. In addition, the oscillation at the frequency of 0.5-3 Hz (which corresponds to 120-20 vertical periods) is easily visible and obstructive to a viewer.